Spread spectrum modulation techniques are being increasingly used in communications systems to provide protection against unauthorized signal detection, interception and exploitation. Spread spectrum modulation also provides a degree of immunity against both unintentional and deliberately introduced interference, e.g., communications fading and jamming. Spread spectrum systems employ a transmission bandwidth that is substantially greater than that of the information-bearing signal.
One common modulation technique used in spread spectrum communication systems is frequency hopping. In a frequency hopping system, the carrier frequency is shifted or switched in a pattern dictated by a prepared code sequence, for example, by a pseudo-random number sequence. The communications receiver must be tuned or matched to the frequency hopping signal, and thus both the transmitter and receiver generate the same code sequence. The receiver is synchronized with the transmitter, for example, by tuning to a first fixed frequency or by having the pseudo-random number related to or derived from the time of day. Thereafter, the receiver duplicates the frequency hops made by the transmitter using the pseudo-random number sequence.
Spread spectrum communication systems that utilize frequency hopping generally use a frequency synthesizer that generates a single frequency for a given digital control word. The makeup of the control word is commonly based on the frequency divider configuration of the synthesizer, and is therefore unique to the particular communications system being implemented. For example, the size of the frequency control word will be influenced by the range and resolution of the frequencies to be used by the system.
Spread spectrum communication systems have been developed where the prepared code sequence outputs digital numbers that correspond to the control words used by the synthesizer. Thus, in this type of system, there is typically a frequency hop for each number sent to the synthesizer. An important consideration in spread spectrum communication systems is the desire to avoid certain frequencies that fall in the range of the synthesizer. Certain frequencies are avoided for a number of reasons, including government testing, use of emergency channels, fading, jamming and other exploitation. The avoidance of unwanted frequencies has typically been accomplished by inhibiting the control words that would select these frequencies. This type of avoidance scheme is undesirable because, for example, at high hop rates, avoiding frequencies by testing each code word generated may require a code word generator to run at excessively high operation rates and with unpredictable timing characteristics.